This invention relates generally to ferroelectric optical devices and, more particularly, to polycrystalline ferroelectric optical modulators.
Ferroelectric perovskite materials are well known for their utility in the optoelectronics art. Single crystal lithium niobate, for example, is used commercially in the design of external electro-optic modulators for laser transmitters. The relatively large electro-optic coefficient of single crystal barium titanate (BTO) makes this material especially well suited to similar applications. See, for example, M. Zgonik et al., Phys. Rev. B, Vol. 50, pp. 59415949 (1994), which is incorporated herein by reference. Others have shown that single crystal thin films of BTO with good structural properties can be grown on magnesium oxide (MgO) and other crystalline substrates using well-known pulsed laser deposition (PLD)techniques. See, respectively, L. Beckers etal., J. AppL. Phys., Vol. 83, No. 6, pp. 3305-3310 (1998) and M. Siegert et al., Mat. Res. Soc. Symp. Proc., Vol. 597, pp. 706-711 (2000), both which are also incorporated herein by reference.
It would be desirable to be able integrate this type of modulator into optical integrated circuits (OICs), in particular OICs fabricated using well-known silicon-optical-bench (SiOB) technology. In this technology, silica optical waveguides are formed on a single crystal silicon substrate. These waveguides are capable of guiding optical radiation at wavelengths of about 1100-1550 nm, and the design typically endeavors to minimize the amount of radiation that is coupled into the silicon substrate, which has a much higher refractive index (about 3.5) than that of silica (about 1.5) at these wavelengths. In one design, an epitaxial MgO optical isolation layer is formed on the silicon substrate to reduce the amount of radiation that is coupled into the substrate. One approach to incorporating an electro-optic modulator into this design would be to deposit, for example, single crystal BTO over the MgO layer. See, U.S. Pat. No. 6,103,008 issued to R. A. McKee et al. on Aug. 15, 2000. However, it is difficult to deposit these layers on a silicon substrate. First, to keep optical losses relatively low the MgO and BTO layers are made to be relatively thick. Second, the thermal expansion coefficients of the layers are different from that of the silicon substrate. Third, the layers are deposited at an elevated temperature, and when they are cooled the combination of thick layers and different thermal expansion coefficients conspires to produce significant strain, which in turn causes the layers to crack.
Thus, a need remains in the art for an optical modulator design that would allow ferroelectric electro-optic materials to be more readily integrated into OICs.
One approach might be to replace the electro-optic single crystalline layer with a polycrystalline layer. This design would simplify integration considerably; the deposition temperature could be lowered and the epitaxial isolation layer could be replaced by an amorphous silicon dioxide layer, thereby alleviating the thickness constraints described above. Nevertheless, this approach is fraught with difficulty too-polycrystalline electro-optic materials tend to have disadvantageously high optical scattering in the wavelength regime where the electro-optic coefficient is high. See, for example, B. Wong et al., J. Appl. Phys., Vol. 70, No. 3, pp. 1180-1184 (1991), regarding the Pockel""s effect in polycrystalline ZnS planar waveguides and E. Dogheche et al., Microelectronic Engineering, Vol. 29, pp. 315-318 (1995) regarding the optical properties of lead-based ferroelectric thin films, both of which are incorporated herein by reference.
In accordance with one aspect of our invention, an optical device comprises a body of ferroelectric material exhibiting an effective electro-optic coefficient (reff) and an optical loss (xcex1), with the body being adapted for the propagation of optical radiation at a wavelength xcexo through it, and means for applying an electric field to the body in order to alter the refractive index therein, characterized in that the body is polycrystalline and has an average grain size such that reff is relatively high and xcex1 is relatively low, both at xcexo. In a preferred embodiment the body has an average grain size that is less than about xcexo/10, preferably in the range of approximately 8-20 nm, which is especially well suited for devices operating at near infrared wavelengths in the range of about 1000-1600 nm. Illustratively, the ferroelectric body is a perovskite material such as BTO or lithium niobate.
As used herein the term ferroelectric material includes paraelectric materials in which the average grain size is so small that the material does not maintain a spontaneous polarization at zero applied field, yet it does exhibit a significant electro-optic coefficient.